easier reads about 20th century history for me and 12 yr old?

Diary of Anne Frank.
Hitler's Last Day Minute by Minute

Animal Farm for the rise of Stalin and communism
When Hitler Stole Pink Rabbit ids great for 1 12 year old
Carrie's War ditto.

Sorry for my terrible typing!!

I'm not sure if these are what you're looking for, exactly but Robert Westall wrote several children's novels about WW2. Titles that spring to mind are The Machine Gunners, Blitz Cat and The Kingdom by the Sea.

They're not so much about the issues - more how living through the war affected people, especially children.

Also, Goodnight Mr Tom by Michelle Magorian.

Hare with amber eyes, very enjoyable book

We Are at War by Simon Garfield - possibly more one for you, OP, though your 12-y-o might like to dip into it. It’s a selection from the diaries kept by various people during wartime for the Mass Observation project (which is still running, in fact). People were asked to write about their daily lives - hundreds signed up to do so. It’s fascinating to read about the home front as the war was actually happening.

Try Maus. It's a graphic novel about the Holocaust and after, written by the son of a survivor about his parents' lives. Very thought provoking, not horrific at all but a good starting point for thinking about and talking about attitudes to past events as well as the events themselves. I studied it as part of an MA course so it is 'pukka' but DS also read it when he was about 10, so I think it will tick your boxes.